The article introduces a free online textbook on compilers and language design, written by Prof. Douglas Thain, which provides a one-semester introduction to compiler construction. The book enables readers to build a simple compiler that accepts a C-like language and translates it into working X86 or ARM assembly language. The textbook is suitable for undergraduate students with experience in programming in C and courses in data structures and computer architecture. The author also provides additional resources, including a GitHub repository with code examples and a mailing list for updates.